Human plasmacytoid dendritic cells (pDC) constitute a rare subset of blood dendritic cells (DC), distinct from myeloid CD11c+, conventional DC (cDC). Through production of high levels of type I IFN in response to virus infection, pDC serve as a critical link between innate and adaptive antiviral immune responses. Recent observations from my laboratory have highlighted their particular role in the immune regulation of HIV-1 infection. pDCs, but not cDCs, undergo activation following CD4 mediated endocytosis of HIV-1 and subsequent activation of TLR7 with genomic RNA. Activated pDCs upregulate costimulatory molecules, produce pro-inflammatory cytokines and chemokines and activate immature cDCs in a bystander fashion. As a counterpoint to the induction of these anti-viral responses, HIV-activated pDCs simultaneously induce the differentiation of Tregulatory cells (Tregs) from naive resting CD4+ T cells, in a TLR7 dependent manner. Treg generation requires the expression of indolamine 2,3-dioxygenase (IDO), an enzyme that catabolizes tryptophan to kynurenine, as it is reversed upon addition of the specific inhibitor 1 methyl-tryptophan. The T regs generated (inducible T regs) inhibit the proliferation of activated T cells and maturation of cDC, thereby attenuating the induction of ongoing adaptive immune responses. Thus pDCs inhibit viral replication and promote anti-viral immunity, but at the same time limit the extent of immune activation. This newly ascribed property of pDCs is especially relevant in HIV infection where control of excessive immune activation could be essential to prevent virus dissemination and progression of disease. In this application we propose to: (1) Identify the mechanism(s) underlying HIV-dependent, pDC-induced T reg differentiation, focusing in particular on IDO; (2) Determine the regulatory processes used by Tregs to inhibit T cell growth and cDC activation; (3) Establish the in vivo relevance of pDC-mediated induction of Treg in HIV-infected patients. These studies will greatly improve our understanding of the events that follow pDC activation by HIV and potentially result in clinically applicable approaches to enhance anti-HIV immune responses in vivo.